This is a very odd problem I've found. I have written and successsfully deployed a VSTO 3.0 Addin for Excel 2007. The problem is that the Tab for the Ribbon only shows up on NEW instances of Excel; the Tab doesn't display if I first open an already existing workbook. However, if I open a new instance of Excel, then open existing workbooks, it'll display.
How can I get the Tab for the Addin to display for both, new instances of Excel and when opening an existing Workbook?